The satellite broadcasting standard referred to as DVB-S2 was developed for the broadcasting of video streams as well as the broadcasting of IP application packets. This standard enables constant, variable or adaptive coding and modulation modes. The structure proposed by this standard involves a concatenation of a BCH (Bose-Chaudluri-Hocquenghem) code and an LDPC (Low Density Parity Check) code. This concatenation is preceded by an energy dispersion step and followed by a QPSK, 8-PSK, 16-APSK or 32-APSK modulation step and a step of inserting a header in π/2 BPSK.
The header contains transmission parameters. The header includes a 5-bit field called MODCOD defining the type of modulation and the coding rate of the frame. This field is previously unknown to the receiver and is protected by a Reed-Müller code (32,5) having the following generating matrix:
         (                            0                          1                          0                          1                          0                          1                          0                          1                          0                          1                          0                          1                          0                          1                          0                          1                          0                          1                          0                          1                          0                          1                          0                          1                          0                          1                          0                          1                          0                          1                          0                          1                                      0                          0                          1                          1                          0                          0                          1                          1                          0                          0                          1                          1                          0                          0                          1                          1                          0                          0                          1                          1                          0                          0                          1                          1                          0                          0                          1                          1                          0                          0                          1                          1                                      0                          0                          0                          0                          1                          1                          1                          1                          0                          0                          0                          0                          1                          1                          1                          1                          0                          0                          0                          0                          1                          1                          1                          1                          0                          0                          0                          0                          1                          1                          1                          1                                      0                          0                          0                          0                          0                          0                          0                          0                          1                          1                          1                          1                          1                          1                          1                          1                          0                          0                          0                          0                          0                          0                          0                          0                          1                          1                          1                          1                          1                          1                          1                          1                                      0                          0                          0                          0                          0                          0                          0                          0                          0                          0                          0                          0                          0                          0                          0                          0                          1                          1                          1                          1                          1                          1                          1                          1                          1                          1                          1                          1                          1                          1                          1                          1                      )  
The header also includes a 2-bit field called TYPE defining the length and the type of frame. This field is previously unknown to the receiver. The header also includes an identification sequence at the beginning of the frame called SOF, previously known to the receiver.
To lock the frame, at each symbol time, the receivers envisaged in the literature compute a correlation level by correlation computations between 26 consecutive sampled symbols. When the correlation level exceeds a threshold, it is considered that these 26 symbols correspond to the SOF identification sequence. A frequency shift error estimation is established on the basis of these symbols and used in a frequency-tracking loop.
Such a receiver has disadvantages. Indeed, the locking performed by the identification of symbols of the SOF sequence can be unreliable when the digital stream is transmitted in a noisy environment. Similarly, the frequency shift error estimation established has a reduced reliability in such an environment. The speed and reliability of the locking are thus insufficient, which is particularly detrimental since, in the absence of decoding of a header, the position of the next headers cannot be determined, as the frames have variable sizes.
Therefore, there is a need for simple means involving a reduced processing power in order to overcome one or more of these disadvantages.